1. Field of the Invention
The present invention relates to partial response maximum likelihood (PRML) code encoding and decoding methods for a high-density data storage device. More particularly, it relates to PRML code encoding and decoding methods for a high-density data storage device, in which digital data is magnetically recorded on and reproduced from a disk memory device with high density without interference between signals.
2. Description of the Related Art
Research has been conducted to more efficiently utilize mass quantities of information in a rapidly developing information society, resulting in great progress in many fields.
In a storing device field, the focus of research has been to transmit large amounts of information, and to reduce the time required to process information, thereby to satisfy information needs in the information-competitive society. That is, efforts are being made to develop rapid transmission of reliable information, while increasing the amount of data recorded (recording density) in a given storage device.
For high-speed, large capacity data storage devices, material aspects are considered in connection with improving the physical characteristics or increasing the accuracy of a storage medium.
Further, signal processing aspects are considered in connection with increasing the recording density of the storage device and facilitating detection of a reproduced signal through efficient encoding and decoding, or reducing data detection errors, relying on signal processing technology.
Studies have been made to increase the amount (recording density) of data recorded on a given storage device during recording and reproducing of data and to rapidly transmit reliable information.
A storing device having data recorded with a high density enables efficient encoding, which in turn reduces redundancy and facilitates signal detection.
However, the problem of intersymbol interference (ISI) becomes more prominent with higher-density recording.
In general, data recorded on a storage device is encoded to run length limited (RLL) codes. The RLL code limits succession of recorded data symbols for timing control of data sampling clock signals and signal detection. That is, in the RLL code, the number of zeros between ones is limited to a minimum of d for easy signal detection and to a maximum of k for timing of data during playback of a signal.
Recently used RLL code encoding methods include code rate 1/2 RLL(2, 7), 2/3 RLL(1, 7), 8/9 RLL(0, 3), and 8/9 RLL(0, 4/4) encoding methods.
The first two encoding methods have d's of 1 and 2, respectively. Thus, they allow one zero and two zeros between ones, respectively, thereby reducing interference between signals.
Despite the advantage of the decrease in signal interference, these methods require many bits to transmit given user data due to a large redundancy caused by their low code rates.
Hence, a low-code rate encoding method causes more intersymbol interferences than a high-code rate encoding method such as the 8/9 RLL(0, 3) and 8/9 RLL(0, 4/4) methods, thus nullifying the advantage of allowance of at least one zero between transitions. The high-code rate encoding method is more favorable for recording and reproducing data due to a smaller redundancy than the low-code rate encoding method.
The high-code rate encoding method increases a channel input SNR, reduces interference between data due to a small redundancy, and enables high-density recording.
In general, a channel should be modeled after an actual one in recording and reproducing data in a storing device. To reflect the channel characteristic of the storing device, channel characteristics can be expressed as (1+D.sup.n) (n=1, 2, . . . ) or (1-D) (1+D).sup.n (n=1, 2, . . . ), where D is delay. Thus, (1+D.sup.n) represents the original data plus n-time delayed data.
In the PRML method, mutually controlled intersymbol interference is set between current data and previous data by precoding an input signal and then data is detected in a Viterbi decoder by modifying a target response d.sub.k to a.sub.k +a.sub.k-1 or a.sub.k -a.sub.k-2, where a is input data, and k is an index.
The PRML method shows excellent detection performance with a recording density of the signal interference given under a channel characteristic of (n=1).
As data is recorded at a higher density, the distance between transitions becomes smaller, thus worsening the intersymbol interference between data. To reduce the interference in high-density recording, the distance between transitions should be increased.
An RLL(1, 7) encoding method, employing this concept, encodes data to have at least one zero between symbols.
However, though the RLL(1, 7) encoding method limits transitions, it has a low code rate.
Therefore, to transmit given data, more bits than the high-code rate encoding methods 8/9 RLL(0, 3) and 8/9 RLL(0, 4/4) are required leading to a small distance between recorded data. As a result, interference between data becomes serious.
That is, a high-code rate encoding method is more useful in recording and reproducing data in channels of a storage device than a low-code rate encoding method.
In addition, the high-code rate encoding method reduces interference between data and non-linearity, relative to the low-code rate encoding method, thus enabling high-density recording.
To reduce the intersymbol interference, there are two schemes: increasing code rate; and setting the interval between transitions.
However, both have a trade-off relationship. That is, it is impossible to set some interval between transitions without reducing the code rate, and the code rate cannot be increased with the interval being kept between transitions.
The 2/3 RLL(1, 7) encoding method relies on the latter method. On the other hand, the 8/9 RLL(0, 3) and 8/9 RLL(0, 4/4) encoding methods are suggested from a different viewpoint. These encoding methods have higher channel SNRs due to their high code rates than low-code rate encoding methods. Further, fewer bits are used than in the low-code rate decoding methods to record given data. Thus, intersymbol interference and thus non-linearity can be reduced.
However, intersymbol interference becomes a serious problem as the recording density of a data storing device is increased.